You have a Formosum Type Dendrobium which under some growing conditions are considered difficult.
Fungus spots may be an indicator that the plant is not happy. If your conditions are not too bad you could try using one of preventative treatments such as echo oil which is based on Canola Oil that puts a preventative film on the leaves to stop the fungus getting in. Once those black scars are there, they are forever even if the fungus is stopped from spreading.

Thanks arthurm

I'm wondering if the fugus is already in the plant. perhaps it's not so clear from this image but it looks like the three newest leaves on one of the new canes are yellowing and drying out?

Sorry, i am not an expert on fungus, but from my experience it is more likely to attack on dewy autumn nights when the fungus spores can penetrate the leaf surface. Ditto, if the plant is growing in unsuitable conditions and its natural resistance is weakened.
When i said that those Formosum Types Dens are difficult I was referring to here. latitude 33S. Over 1000 orchids exhibited by members of a local orchid society in the last year and nil Formosum Type Dens.
Check the net for culture notes to see if your conditions are close to those recommended. The plant doesn't look in bad condition but take note that i have tried to grow these beauties several times and they died Sigh!
Hopefully, someone with more success will answer your question